The invention is related to the field of fixed cutter bits used to drill wellbores through earth formations. More specifically, the invention is related to structures for, and methods for making, alloy bodies for polycrystalline diamond compact cutter drill bits.
Fixed cutter drill bits known in the art include polycrystalline diamond compact (PDC) bits. The typical PDC bit includes a bit body which is made from powdered tungsten carbide infiltrated with a binder alloy within a suitable mold form. The particular materials used to form PDC bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. The cutting elements used on these bits are typically formed from a cylindrical tungsten carbide xe2x80x9cblankxe2x80x9d or substrate. A diamond xe2x80x9ctablexe2x80x9d made from various forms of natural and/or synthetic diamond is affixed to the substrate. The substrate is then generally brazed or otherwise bonded to the bit body in a selected position on the surface of the body.
The materials used to form PDC bit bodies, in order to be resistant to wear, are very hard and are therefore difficult to machine. Therefore, the selected positions at which the PDC cutting elements are to be affixed to the bit body are typically formed substantially to their final shape during the bit body molding process. A common practice in molding PDC bit bodies is to include in the mold at each of the to-be-formed cutter mounting positions, a shaping element called a xe2x80x9cdisplacementxe2x80x9d. A displacement is generally a small cylinder made from graphite or other heat resistant material which is affixed to the inside of the mold at each of the places where a PDC cutter is to be located on the finished drill bit. The displacement forms the shape of the cutter mounting positions during the bit body molding process. See, for example, U.S. Pat. No. 5,662,183 issued to Fang for a description of the infiltration molding process using displacements.
PDC bits known in the art have been subject to fracture failure of the diamond table, and/or separation of the diamond table from the substrate during drilling operations. One reason for such failures is compressive contact between the exterior of the diamond table and the proximate surface of the bit body under drilling loading conditions. One solution to this problem known in the art is to mount the cutting elements so that substantially all of the thickness of the diamond table is projected outward past the surface of the bit body. While this solution does reduce the incidence of diamond table failure, having the diamond tables extend outwardly past the bit body can cause erratic or turbulent flow of drilling fluid past the cutting elements on the bit. This turbulent flow has been known to cause the cutter mounting to erode, and to cause the bonding between the cutters and the bit body to fail, among other deficiencies in this type of PDC bit configuration. It is preferable to have the PDC cutters mounted so that they are substantially flush with the outer surface of the mounting position on the bit body.
What is needed is a structure for a PDC bit body which reduces diamond table failure, while retaining the benefits of flush mounting of the cutters on the bit body.
One aspect of the invention is a method for forming a drill bit body which comprises infiltrating powdered tungsten carbide with a binder alloy in a mold. The mold has therein at least one displacement adapted to form a mounting pad for a cutting element. The displacement comprises a substantially cylindrical body having a diameter selected to substantially conform to a radius of the cutting element and a projection adapted to form a relief groove under a position of a diamond table forming part of the cutting element when the cutting element is mounted on the pad. The width of the relief groove is selected so that the relief groove extends back from an outer surface of the bit body at least about 40 percent of that portion of a thickness of the diamond table which does not extend past the outer surface.
Another aspect of the invention is a drill bit body comprising a main body having at least one blade formed therein, and at least one cutting element mounting pad formed on the at least one blade. The mounting pad is adapted to receive therein a substrate of a cutting element. The mounting pad has a relief groove therein under a position of a diamond table in the cutting element when the cutting element is mounted on the pad. The width of the relief groove is selected so that the relief groove extends back from an outer surface of the blade at least about 40 percent of that portion of a thickness of the diamond table which does not extend past the outer surface.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.